Due to the increasing importance of spectral efficiency in wireless communications systems, linearity and efficiency of radio frequency (RF) power amplifiers have become critical design issues, particularly for implementing complex digital modulation schemes often necessary for supporting higher data capacity and enhanced data transmission rates. Although linearity can be improved with known linearization techniques such as feed-forward linearization, this improvement comes at the expense of reduced amplifier efficiency.
Doherty amplifiers, which are well-known in the art, have been shown to achieve higher efficiencies than traditional power amplifier designs. A standard Doherty amplifier 100 is illustrated in FIG. 1. As apparent from the figure, the standard Doherty amplifier 100 consists of a carrier amplifier 102 and a peak amplifier 104 which are typically biased in class A and in class C, respectively. A quadrature 3-decibel (dB) hybrid 106 can be employed to split an input signal applied to the Doherty amplifier 100 equally, but 90 degrees out of phase, to both the carrier and peak amplifiers. Amplified output signals generated by the carrier and peak amplifiers 102, 104 are combined in phase at an output of a quarter-wave transformer 108 which is coupled to an output of the carrier amplifier.
Operation of the Doherty amplifier can be separated into two primary regions. In the first region, the input power is less than a threshold of the peak amplifier 104, and therefore only the carrier amplifier 102 supplies the output power to an output load RL, connected to an output of the Doherty amplifier 100, with an efficiency determined primarily by its class A operation. As the input signal further increases to a level just below a saturation point of the carrier amplifier 102, the peak amplifier 104 begins to operate, marking the start of the second region of operation. Through the connection of the quarter-wave transformer 108, the power supplied by the peak amplifier 104 effectively reduces the apparent load impedance seen by the carrier amplifier 102. This impedance reduction enables the carrier amplifier 102 to deliver more power to the output load 110 while its voltage remains saturated. In this manner, a higher efficiency is maintained in the carrier amplifier 102, and hence the overall Doherty amplifier 100, throughout the second region until the peak amplifier 104 reaches its saturation threshold.
Although Doherty amplifiers can generally achieve higher efficiencies than traditional power amplifier designs, this increased efficiency comes at the expense of reduced linearity. This is due, at least in part, to the fact that Doherty amplifiers typically employ fixed electrical line lengths and/or phase shift elements to achieve proper phasing between the two signal paths. These fixed line lengths have static phase characteristics associated therewith which vary nonlinearly with frequency. This reduced linearity significantly limits the bandwidth of conventional Doherty amplifiers.
There exists a need, therefore, for a power amplifier having an extended frequency range of operation compared to traditional power amplifiers, which does not suffer from one or more of the problems exhibited by conventional power amplifiers.